Wavelength Division Multiplexing (WDM) is a method by which optical fibers are used to carry multiple light waves of different frequencies. In a WDM network many wavelengths are combined in a single fiber, thereby increasing the carrying capacity of the fiber. Signals are assigned to specific frequencies of light (wavelengths) within a frequency band. This multiplexing of optical wavelengths is analogous to the way radio stations broadcast on different wavelengths as to not interfere with each other. Because each radio channel is transmitted on a different wavelength, a desired channel may be selected using a tuner. WDM channels (wavelengths) are selected in a similar manner. In a WDM network, all wavelengths are transmitted through a fiber, and demultiplexed at a receiving end. The fiber's capacity is an aggregate of the transmitted wavelengths, each wavelength having its own dedicated bandwidth. Dense Wavelength Division Multiplexing (DWDM) is a WDM network in which wavelengths are spaced more closely than in a coarse WDM network. This provides for a greater overall capacity of the fiber.
Modern networks use WDM, including coarse WDM (CWDM) and dense WDM (DWDM), to increase the amount of traffic that can be transmitted through the network. WDM signals may propagate through optical networks, in both clockwise and counterclockwise directions, connecting each node pair via two paths. Alternatively, WDM signals may propagate through the network in only one direction, limiting each node pair to a single connection path.
Many WDM networks use Reconfigurable Optical Add/Drop Multiplexers (ROADMs) to add or drop traffic to or from the network. Selected wavelengths can be added or dropped using the ROADMs by issuing commands from a central Network Management System (NMS). Typically, a ROADM deployment scenario exists where bandwidth needs to be deployed between two locations. Normally, a service provider installs transponders at the locations, balances the power across each span, and starts using the service. This basic approach is much simpler when the network is first installed and an abundance of wavelengths is available. As more connections over the network are created, however, more wavelengths will be needed because, for a given connection, the same wavelength typically needs to be available on every span of the path supporting the connection over the network. Further, if the service provider is planning a resilient service, it must ensure that two contiguous wavelengths are available between each start and end point of each communications path. If a contiguous wavelength between the two locations is not available, then an optical connection between the locations typically may not be established.